1. Field of the Invention
This invention pertains to the manufacture of metal workpieces, and more particularly, it relates to hot isostatic pressing of metal workpieces.
2. Description of the Prior Art
In the manufacture of metal workpieces, especially those formed by the casting of ferrous metal (e.g. steel), improvements in the mechanical properties are necessary for a satisfactory product. For example, the casting, at a temperature above the plastic range of the metal, is subjected to mechanical working. For cast steel, this temperature is at least 2000.degree. F. The mechanical working can be by rolling, forging, pressing and other mechanical pressure applications that effect structural changes that improve the mechanical properties of the workpiece. For example, hot mechanical working reduces the internal defects of steel products attributed to segregation, cracks, seams, and inclusions. The term ferrous metal includes all of its forms such as wrought iron and steel.
It has been proposed to employ hot isostatic pressing of metal workpieces to obtain similar internal improvements as can be obtained by hot mechanical working. In hot isostatic pressing, the metal workpiece, while at a temperature in the plastic range is subjected uniformly on all three axis to superatmospheric gas pressure (e.g. 20,000 psi) until the internal mechanical properties are improved, especially the reduction of internal voids. However, the problems in maintaining the metal workpiece at the necessary elevated temperature during isostatic pressure application has prevented satisfactory application of the hot isostatic pressing procedures especially to complex castings. The extremely high pressures prevent continuous heating of the workpiece during the pressing step.
It is the purpose of this invention to provide an improved process of hot isostatic pressing wherein the metal workpiece remains uniformly throughout at a uniform temperature in the plastic range of the metal.